1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method, and more specifically, to an ink jet printing apparatus and an ink jet printing method using pigment ink.
2. Description of the Related Art
More and more ink jet printing apparatuses such as printers, facsimile machines, and copiers which utilize an ink jet printing method have been rapidly placed on the market.
These ink jet printing apparatuses include a print head from which ink droplets are ejected and an ink tank from which ink is supplied to the print head. For these ink jet printing apparatuses in which the print head and the ink tank are integrated together, when the ink in the ink tank is used up, the print head must also be replaced with a new one. Accordingly, a large number of supplies are required, thus increasing running costs.
Thus, in most of the recent ink jet printing apparatuses, the print head and the ink tank are separated from each other so that when the ink in the ink tank is used up, only the ink tank needs to be replaced with a new one. In this case, the running costs decrease compared to the configuration in which the print head and the ink tank are integrated together. However, to further reduce the running costs, the capacity of the ink tank must be increased. Further, when a printing medium (a sheet) of an International Organization for Standardization size A0, B0 or the like is printed on by a large-sized printer, a large amount of ink is used to print one sheet. In this case, the ink tank must be more frequently replaced with a new one unless the number of sheets that can be printed per ink tank is somewhat large. Consequently, the ink tank must have a large capacity.
As the capacity of the ink tank increases, it is necessary to increase the size and weight of a carriage on which the print head and the ink tank are installed to carry out serial scan for printing. When the carriage is heavy, it is difficult to cause the carriage to carry out scan at high speed. In this case, scanning at high speed requires the use of an expensive high-torque motor. This increases the costs of the apparatus.
Thus, a printing apparatus has been put to practical use in which the ink tank is not mounted on the carriage, which carries out scanning, but is fixed in a predetermined area in the ink jet printing apparatus, with only the print head mounted on the carriage and with the print head and the ink tank joined together via a tube or the like. In this apparatus, since only the print head is mounted on the carriage, this apparatus is light and can carry out scanning at high speed, although it is required that the print head and the ink tank are joined together via a tube or the like. Further, the capacity of the ink tank can be increased as required. Accordingly, the capacity may be set taking into account the running costs, the size of the apparatus, the frequency of ink tank replacements, and the like.
FIG. 7 schematically shows a configuration of a printing apparatus in which a print head and an ink tank are joined together via a tube. Reference numeral 1 denotes a print head mounted on a carriage (not shown), and a reference numeral 5 denotes an ink tank. Reference numeral 3 denotes a supply tube that joins the print head 1 and the ink tank 5 together, and reference numeral 4 denotes an atmospheric communication tube. When a printing operation is performed using the print head 1, the ink in the ink tank 5 is fed from the ink tank 5 to the print head 1 via the supply tube 3. On the other hand, an amount of air corresponding to an amount of ink supplied is introduced into the ink tank through the atmospheric communication tube 4. With this configuration, the size of the ink tank can be freely set according to the application. Accordingly, a large ink tank can be designed when it is expected that a large amount of ink is used as in the case with large-sized printers or network printers.
As an application of the large-sized printer, there has been a growing demand for printing of materials intended for outdoor display such as posters. Materials intended for outdoor display (printing sheets) require weathering resistance, but dye ink commonly used for conventional ink jet printing apparatuses has insufficient weathering resistance. In particular, relating to light or gas resistance, the dye ink may change over time and may thus be significantly discolored. Although the dye ink has been improved to exhibit higher weathering resistance under relatively favorable conditions, e.g., indoors, this weathering resistance is still far from the outdoor use level. For large-sized sheets, materials intended for outdoor display may comprise a laminate applied to a printed sheet. In this case, however, although the gas resistance is improved, the light resistance is not at a practical level.
Thus, apparatuses have been put to practical use which employ pigment ink instead of dye ink. Normally, the dye ink is a solution that the dye is dissolved thereinto, whereas the pigment is a solution that the pigment particles are dispersed therein. The long-term dispersion stability of the pigment particles is a very important factor of printing apparatuses that can always provide very reliable high-grade outputs. The dispersion stability of the pigment markedly affects the type or composition of a dispersant such as resin or an activator which is used for dispersion. However, it is difficult to obtain the pigment ink that remains unchanged over a long period. Normally, if the pigment ink is left over a long period, the density of the pigment in the pigment ink will be nonuniform. For example, the density of the pigment may vary between the upper and lower parts of the ink tank. If the apparatus is used under such conditions, the resulting density differs from that obtained if the pigment is uniformly dispersed.
If the apparatus has been out of use over a long period, the dispersability can be improved to some degree by performing a recovery operation with a pump or the like. However, this improvement is not sufficient. Further, the recovery operation may waste a large amount of ink to increase the running costs. Further, the amount of waste ink increases, thus requiring the printing apparatus to have a larger capacity for containing this waste ink. This increases the size and costs of the apparatus.
Thus, the applicant proposed a printing apparatus in which if the apparatus has been out of use over a long period, an ink flow is formed by operating a valve provided in a tube through which ink is supplied and thereby a variation in density within the ink tank is suppressed.
This printing apparatus can make the density in the ink tank uniform but is insufficient to suppress a variation in density within the tube or print head. It is difficult to make the density in the print head uniform simply by carrying out preliminary ejection before the start of printing. Even if dense ink located close to nozzles can be ejected, thinner ink remains close to the print head. Consequently, the resulting printed image has a low grade.
Further, Japanese Patent Application Laid-Open No. 8-281967 (1996) discloses an apparatus in which the supply tube is improved or a projection is provided in the supply tube, in order to suppress a variation in the density of the ink in the tube. Such an apparatus may reduce the degree of freedom for design. It also prevents the use of ordinary tubes, thus complicating the apparatus and increasing the costs.
Thus, the present invention is provided to solve these problems. It is an object of the present invention to provide an ink jet printing apparatus and an ink jet printing method which always provide very reliable high-grade outputs by suppressing a variation in the density of ink within an ink supply passage or print head without performing any useless recovery operations and complicating the configuration of the supply tube.